The development of artificial photosynthesis technology that replicates photosynthesis of plants to electrochemically convert sunlight to a chemical substance has been recently progressing in consideration of an energy problem and an environmental problem. Converting sunlight to a chemical substance to store it in a cylinder or a tank is advantageous in that it costs lower for energy storage and has a less storage loss than converting sunlight to electricity to store it in a battery.
A known electrochemical reaction device that electrochemically converts sunlight to a chemical substance is, for example, a two-electrode device including an electrode having a reduction catalyst for reducing carbon dioxide (CO2) and an electrode having an oxidation catalyst for oxidizing water (H2O), the electrodes being immersed in water containing dissolved carbon dioxide. In this case, these electrodes are electrically connected to each other via an electric wire or the like. The electrode having the oxidation catalyst oxidizes H2O using light energy to produce oxygen (½O2) and obtain a potential. The electrode having the reduction catalyst obtains the potential from the electrode that causes the oxidation reaction, thereby reducing carbon dioxide to produce formic acid (HCOOH) or the like. Such two-stage excitation for obtaining the reduction potential of the carbon dioxide makes the two-electrode device low in conversion efficiency from the sunlight to the chemical energy.
An electrochemical reaction device including a stack (silicon solar cell or the like) of a pair of electrodes and a photoelectric conversion layer sandwiched therebetween is also under investigation. The electrode on a light irradiated side oxidizes water (2H2O) using light energy to produce oxygen (O2) and hydrogen ions (4H|). The electrode on the opposite side uses the hydrogen ions (4H+) produced in the electrode on the light irradiated side and a potential (e−) generated in the photoelectric conversion layer to produce hydrogen (2H2) or the like as a chemical substance. An electrochemical reaction device including a stack of silicon solar cells is also known. The aforesaid electrochemical reaction device preferably has high conversion efficiency.